The Bacteroides are one of the numerically dominant genera of the human intestinal microbiota where they are mutualistic symbionts providing beneficial properties to humans. Outside of the natural colonic niche, Bacteroides fragilis is an opportunistic pathogen and is the leading cause of anaerobic bacteremia and intraabdominal abscesses. B. fragilis synthesizes eight capsular polysaccharides per strain, which are instrumental in the ability of this organism to both provide symbiotic benefits in its natural intestinal niche, and to cause disease if the bacterium gains access to extraintestinal sites. Our long-term objective is to understand the importance of these capsular polysaccharides in both the intestine and extraintestinal sites and their importance to both processes. We have made extensive progress in understanding the molecular mechanisms governing the complex regulation of these polysaccharides; however, we still lack a fundamental understanding of why this organism has evolved to synthesize eight distinct capsular polysaccharides and phase vary their expression. In this application, we will apply the extensive data we have accumulated regarding regulation of these surface molecules to begin to answer this very important biological question. For Aim 1, we will determine why the bacteria need to be encapsulated in order to colonize the gnotobiotic mouse intestine. We will also determine if mutants expressing each of the eight capsular polysaccharides singly are equivalent in their ability to competitively compete with wild type for intestinal colonization. In Aim 2, we will use both in vitro and in vivo assays to begin to address the biological significance behind why these organisms have evolved to synthesize an extensive number of capsular polysaccharides and coordinately regulate and phase vary their expression. In Aim 3, we will directly analyze polysaccharide expression profiles from bacteria isolated from different environments representing both symbiosis and disease. For these analyses, we will obtain human fecal and clinical samples containing B. fragilis to analyze the bacteria directly from these relevant samples. Completion of these aims will greatly contribute to our understanding of these important bacteria of our intestinal microbiota. These experiments will allow us to determine why these organisms evolved this intriguing biological property and its contribution to human health and disease.